FlexSim 3D Ecological modelling made user friendly Janus Larsen Karen Timmermann Christian Mohn FlexSim 3D Ecological modelling made user friendly The modeling framework FlexSim is a fast, flexible and very user friendly tool specifically targeted towards scientific and management challenges of the complex biogeochemical processes in coastal zone ecosystems. FlexSim is a modular marine modeling framework, which contains a simplified formulation of estuarine hydrodynamics and more complex descriptions of pelagic and benthic biogeochemical processes. It has a very short computational time and can be easily adapted to different areas. FlexSim allows the user to set up computational grids of different complexity ranging from simple box model configurations to high-resolution unstructured grids. It can be applied by non-experts after a short introduction and might therefore be a good candidate to support managers and scientists without the need of an extensive computing infrastructure. Prototypes of FlexSim have been successfully tested for several Danish estuaries. Here, we present FlexSim and modeling results of the physical conditions in Horsens Fjord, an estuary on the east coast of Jutland, Denmark. Model simulations were carried out for different scenarios based on varying complexity of grid geometry and resolution. The model results were compared with observations from the Danish National Monitoring Programme. Computational grids Computational grids and bathymetry in color codes for Horsens Fjord used in this study. Erosion and build-up of stratification Snapshot of surface salinity and position of transect of the vertical section shown in the figures below. Vertical salinity distribution as a response to individual wind forcing conditions (indicated by the blue vertical line in the wind speed time series above each figure). HD model results Observed (points) and modeled (lines) surface (full/filled) and bottom salinity (dashed/open) 2004 time series for the box model setup and high-resolution unstructured mesh configuration. 2004 forcing data for wind speed and fresh water volume discharge through the rivers are shown in the bottom plot. Tracing the source water A tracer was added to the model. First of Feb, May, Aug and Nov it was set to 1 for 24 hours. The plot to the left show the tracer concentration near the source and near the boundary in the following days. The peak concentration at the boundary occurs 2-8 days after tracer release, depending on the amount of fresh water run-off in period. Below the Feb event is depicted as an animation with the surface concentration at the top and a section from source to boundary at the bottom. Future and Funding FlexSIM has been developed by the Baltic Nest Institute (BNI) at Department of Bioscience, Aarhus University. We are currently applying for additional funding to continue developing the system. The development plans include support for decoupling pelagic and benthic ecological models from full 3D hydrodynamic model fields provided by any hydrodynamic model, further refinement of the implemented ecological model templates and implementation of 3D benthic models.